1. Field of the Invention
The present invention relates to data processing systems and more particularly to data processing systems made up of a plurality of loosely-coupled stand-alone systems.
2. Description of the Prior Art: FIG. 1
It has been common in the art to make large computer systems by combining a number of smaller computer systems. The components thus assembled into a large computer system may be either tightly or loosely coupled together. FIG. 1 contains a conceptual block diagram of a tightly-coupled system 101. System 101 contains at least a number of mass storage devices 103, a number of CPUs 105, and a number of memory units 109. All of these devices are connected to a single bus 107 and function as a single unit. Thus, any of the CPUs 105 may reference data in any of MEMs 109 and may retrieve files from any store 103. Since system 101 is in fact a single system, users of system 101 need know nothing about the system's topography.
FIG. 1 further contains a conceptual block diagram of loosely-coupled system 111. Loosely-coupled system 111 consists of a number of stand-along computer systems 113 connected by a network 115. Mass storage units 115 may be associated with any of the stand-along systems 113. Data may be exchanged between systems 113 via network 115, but system 111 does not appear to its users as a single system. If a user on system 113(a) wishes to retrieve a file from store 115 connected to system 113(n), he must specify that the file is available at system 113(n). Thus, unlike the user of system 101, the user of system 111 must know where a resource is located in the network in order to use it.
System 101 and System 111 both have advantages and disadvantages. Tightly-coupled system 101 is in fact a single computer system, and places no more burden on its users than any other single computer system. While convenient to use, system 101 has a number of disadvantages. Among them are the fact that a system 101 cannot be made by linking preexisting computer systems together. A user who wishes to have the increased capacity and ease of use offered by such a system must purchase an entirely new system. Another disadvantage is the fact that the failure of a single component of system 101 such as bus 107 may make the system useless for all users. Further, there may be small fixed limits to the number of CPUs 105 or memories 109 which may be attached to tightly-coupled system 101, and consequently, it may not be possible to expand system 101 beyond a given maximum size.
System 111 is not as easy to use as system 101, since the user must be aware of the topography of system 11 and of which system 113 has a resource to use resources form systems 113 other than the one he is using. On the other hand, system 111 may be put together using systems 113 already available to the user and may be expanded simply by hooking additional systems 113 to network 115. Moreover, system 111 is not dependent on any single component. If a given system 113 fails, only those using that system or data stored thereon are affected, and if network 115 fails, users of each of the systems 113 may continue working locally.
As may be seen from the foregoing survey of the prior art, what is needed, and what is provided by the present invention, is a system which combines the ease of use of tightly-coupled system 101 with the economy, robustness, and ease of expansion of loosely-coupled system 111.